The U.S. Justice Department has withdrawn a series of allegations made in federal court that tie Binyam Mohammed, a British resident held at Guantanamo Bay, to a plot to explode a radioactive "dirty bomb" in the United States, blow up apartment buildings here and release cyanide gas in nightclubs.

Defense lawyers said the decision should force the Pentagon to drop charges of conspiracy and material support for terrorism against Mohammed, which were filed by military prosecutors in May. The charges, the lawyers said, are spurious and based on false confessions obtained through torture.

They said the Justice Department dropped key allegations to avoid having to turn over evidence of abuse. The agency did not respond to a request for comment.

The dirty-bomb allegation was also never pursued in the case of Mohammed's alleged co-conspirator, Jose Padilla, a U.S. citizen initially declared an enemy combatant but convicted in federal court in August 2007 on a lesser charge of providing material support for terrorism. He was sentenced to 17 years in prison.

"There are no serious, hard charges against Mohammed," said Air Force Lt. Col. Yvonne R. Bradley, his military attorney. "The whole thing the government was hanging its hat on, pursuing Mr. Mohammed, was the dirty bomb."